Laserdiode

A laser diode ( semiconductor laser also ) is the light-emitting diode (LED) related semiconductor device, however, the generated laser radiation.

In the laser diode, a pn junction with high doping is operated at high current densities. The choice of the semiconductor material determines the emitted wavelength, wherein a spectrum of infrared and ultraviolet covered today.

History

The idea of ​​using a semiconductor diode as a laser, was followed by the appearance of the first laser in 1960 and even earlier by various physicists. In the early 1960s several laboratories fought a race to build the first semiconductor laser Robert N. Hall of General Electric ( Schenectady ), Nick Holonyak of General Electric (Syracuse ), Marshall Nathan from IBM and Robert Rediker from the Lincoln Laboratory of the Massachusetts Institute of Technology ( where development under the leadership of Benjamin Lax state). They were based on gallium arsenide and had in common that they were still not very efficient, running only in pulse mode and only operated at liquid nitrogen cooling. In September 1962, the team managed by Hall by a narrow margin, to bring the first semiconductor laser to run ( in the infrared at 850 nm, Holonyak demonstrated shortly after the first semiconductor laser in the range of visible light ). In Russia, this is 1963, a team under Nikolai Basow. Practicable semiconductor laser developed only after Herbert Kroemer in the U.S. and Zhores Alferov and Rudolf Kasarinow in the Soviet Union ( Joffe Institute) 1963, the use of thin layers in a sandwich arrangement ( heterostructures ) suggested ( Alferov and Kroemer received for the 2000 Nobel Prize in Physics ). Again, there was a race between Russians and Americans, 1970 brought teams at Bell Laboratories ( Morton Panish, Izuo Hayashi ) and the Joffe Institute ( Alferov ) continuous semiconductor laser at room temperature for running, with the Joffe Institute earlier came to their destination.

Function

The emission of light is caused by recombination of electrons and holes at the interface between p- and n- doped region. The end faces of the component are partially reflective, thus forming an optical resonator in which a standing wave of light can be formed. If there is a population inversion, stimulated emission may be the dominant radiation process. The laser diode emits laser radiation then.

The generation of population inversion occurs in laser diodes by electrical pumps, a direct electrical current in the forward direction ensures steady supply of electrons and holes. The pump current in which the laser operation is used, also called laser threshold, or the threshold current Ith ( ENGL. Threshold).

Construction

Most laser diodes are edge-emitting (English emitter edge ), i.e., the light exiting the crystal to its breaking edge close to the surface transverse to the current. The power loss depending on the wavelength of 30% to 80%, the crystal is heated and must be removed by a suitable cooling. For medium power (500 mW ) are used heatsink, at higher average powers are heat pipes and liquid cooling are used.

The risk of overheating is a limiting factor for the achievable power per single radiation emitter dar. order to achieve a higher performance be used in a strip-shaped chip adjacent a plurality of diodes connected electrically in parallel. By combining the individual beams can achieve higher overall performance. Such an arrangement of a plurality of adjacently on-chip diodes as bars called (english bar). The 10 to 25 single emitter of an ingot behave electrically equal on the basis of the common manufacturing process and can be used in parallel as a larger diode can be operated. You can reach so at currents up to 80 A optical power up to 100 watts in the near infrared.

Made up of several such bars stack and diode lasers made ​​from these materials reach powers in the kilowatt range. Surface emitter (English VCSEL) have lower benefits, but a better beam quality.

State of the art

The efficiency of a laser diode, defined as the ratio of the radiated power to the consumed power. The value of the light yield makes for diodes that emit invisible infrared or in the ultraviolet range, makes little sense.

The achievable efficiency in 2011 was between 10% (green, 530-540 nm ), 20% (blue, 440 nm ) and 70% (red and IR, from 650 nm). 2012 blue laser diodes reached 27 % at a power consumption of 1.4 W in a TO -56 package ( 5.6 mm ) with a lifespan of 10,000 hours. The preparation of suitable InGaN semiconductor materials for green lasers, which tolerate a high current density, is still problematic. For lighting purposes, so it is cheaper to encourage short-wavelength blue light suitable phosphors in the longer wavelength range.

Electrical actuation

Laser diodes are often placed with a photodiode together in a common housing.

Here, the photodiode in this case referred to as a monitor diode optically coupled to the laser diode. It serves as a sensor in a control circuit to keep the optical power of the laser diode constant by an external electronic circuit.

The additional photodiode, the case of laser diodes, such as those used in CD players and laser pointers, three terminals, as outlined in the attached drawing of an exemplary electrical configuration of the two diodes.

Laser diodes only tolerate low reverse voltages in the range of 3-5 V. Furthermore, they are sensitive to electrostatic discharge and are used to transport usually shorted. When handling and installing protective measures must be taken to prevent electrical voltages between the terminals.

Typical parameters and characteristics

A single emitter is about 0.1 mm high, 0.5-2 mm long and 0.5-1 mm wide, with the active zone is less than 1 micrometer high.

The emitted light power, depending on diode type a few hundred microwatts to over 10 watts per single emitter. The power required for this is about 0.1 to 12 A per emitter, the voltage is at infrared laser diodes 1.8-2.2 V. In the pulsed mode (so-called q- cw mode ) can still achieve greater benefits. The modulation frequencies can be up to 10 GHz.

Laser diodes can (laser light of a plurality of different modes of vibration at the same time ) and in the single-mode operation (only one vibrational mode ) work in either multi -mode operation. If single-mode operation is necessary for an application, this can be done by patterning the semiconductor material as with DFB (english distributed feedback laser ) or DBR laser (english distributed Bragg reflector laser ), or an additional external resonator (English External cavity diode laser, ECDL) are obtained: As for other lasers may also be in the laser diode of the optical resonator extend beyond the length of the active semiconductor addition, however, the length can be small due to the divergence, is complicated by the high refractive index of the semiconductor material which already leads to the exit surface to a high reflection.

The frequency of the light emitted from the laser diode is dependent on the temperature, the pump current, and if necessary, the optical feedback from an external resonator except the material. By stabilization of these parameters, a band width of the emitted light of less than one megahertz can be achieved.

Through the pump occurs at a periodic change of the refractive index in the semiconductor material, because this is highly dependent on the charge carrier density. The change in refractive index corresponds to a variation of the optical length of the resonator while maintaining the same geometrical length of the resonator. Thus, the wavelength changes, i.e., the laser changes its emission wavelength.

The heating of the laser leads to wavelength changes. The displacement is approximately 0.25-0.3 nm / K, and the maximum of the radiation shifts when heated by reducing the band gap toward a longer wavelength.

The fracture surface ( facet) is extremely sensitive to contamination, as in the field of radiation leakage from the narrow active zone there is a very high radiation flux densities. At large current pulses can cause even without pollution to optically induced thermal destruction of the facet even there. This type of destruction is called COD ( catastrophic optical damage English, German " catastrophic optical damage ").

Applications

Commercially available wavelengths of semiconductor lasers, and their applications are:

• 405 nm - Based on the semiconductor material indium gallium nitride. Blue - violet laser, used in Blu- ray Disc and HD - DVD drives.
• 445 nm - use as light sources in video projectors.
• 515 nm - Based on the semiconductor material indium gallium nitride. Still in the laboratory stage.
• 531 nm - Based on the semiconductor material indium gallium nitride. Still in the laboratory stage.
• 635 nm - Good quality red laser pointer. Application also for the optical measurement in LIDAR.
• 650 nm - DVD drives, laser pointers
• 670 nm - Low-quality and low-cost red laser pointer. Also be used with barcode readers.
• 760 nm - Gas spectroscopy: oxygen.
• 780 nm - Compact Disc drives, laser printers, light barriers.
• 808 nm - pump lasers ( DPSS ) Nd: YAG laser. Applications include pump laser for green laser pointers or diode lasers and their arrays.
• 980 nm - pump lasers ( DPSS ) Nd: YAG laser.
• 1064 nm - applications in fiber optic networks for data transmission.
• 1310 nm - applications in fiber optic networks for data transmission.
• 1480 nm - of pump lasers ( DPSS ) Nd: YAG laser.
• 1512 nm - Gas Spectroscopy: ammonia.
• 1550 nm - applications in fiber optic networks for data transmission.
• 1625 nm - applications in fiber optic networks for data transmission. In the context of wavelength division multiplexing (WDM ) is typically used for the service channel for network control
• 1654 nm - Gas Spectroscopy: methane.
• 1877 nm - Gas spectroscopy: water vapor.
• 2004 nm - Gas spectroscopy: carbon dioxide.
• 2330 nm - Gas spectroscopy: Carbon monoxide.
• 2680 nm - Gas spectroscopy: carbon dioxide.
• 3030 nm - Gas Spectroscopy: ethyne.
• 3330 nm - Gas Spectroscopy: methane.

For more general applications are:

• In the scientific field, especially in spectroscopy ( TDLAS ), chemical analysis, trace analysis and quantum optics
• For exposure in the printing art, as a sensor element for computer mice (approx. 832-865 nm).